Secret Teachings of All Ages, by Manly P. Hall on pacts with the Devil:
“In the case of black magic, it is the magician and not the demon who must sign the pact. When the black magician binds an elemental to his service, a battle of wits ensues, which the demon eventually wins. With his own blood the magician signs the pact between himself and the demon, for in the arcanum of magic it is declared that ‘he controls the soul who controls the blood of another.’ As long as the magician does not fail, the elemental will fulfil to the letter his obligation under the pact, but the demon will try in every possible way to prevent the magician from carrying out his part of the contract.”
Pasted from <http://www.sacred-texts.com/eso/sta/sta24.htm> THE SECRET TEACHINGS OF ALL AGESby Manly P. HallAt <http://www.sacred-texts.com/eso/sta/>